Spirooxindole frameworks are common in bioactive natural products and pharmaceutical leads. The spirooxindole alkaloids such as mitraphylline, pteropodine, speciophylline and uncarine F are a class of natural products containing a priviledged bicyclic architecture. The development of efficient synthetic strategies to access spirooxindole motifs has been a challenge. The demand for concise asymmetric methods that provide a set of highly enantiomerically enriched spirooxindoles is high. Although syntheses of many types of spirooxindoles, including spirooxindole-derived cyclohexanones and lactols, have been reported, there are groups of spirooxindole motifs of interest that have not been efficiently and/or enantioselectively synthesized. The spirooxindole tetrahydropyranones (see Non-Patent Literature 1) are one such group. Tetrahydropyranones are important core structures and they can be transformed to substituted tetrahydropyrans and related derivatives, spirooxindole tetrahydropyranones should also be useful for further diversification.
Formal hetero-Diels-Alder (hDA) reactions, including highly enantioselective versions, are common routes to tetrahydropyranones. Most of the reported hDA reactions that afford tetrahydropyranones, however, use silyl enol ether-derived dienes or siloxybutadiene derivatives as dienes whether the catalysts are metal catalysts (see Non-Patent Literature 2) or hydrogen bonding-providing catalysts (see Non-Patent Literature 3).